Analysis of Stratigraphic Barriers (Caprock) Between Sands in the Cretaceous Mesa Verde Formation, US DOE Multiwell Experiment, Garfield County, Colorado

摘要

Pressure differences between the caprock tests performed and the in-situ reservoir pressures at the Multiwell site preclude any conclusions as to whether drawdown of one sandstone could result in production from an adjacent sand by gas breaching the stratigraphic barriers. On the other hand, the caprock tests performed at IGT do provide the following useful conclusions: (1) In-situ water permeability of the stratigraphic barrier material is extremely low. (2) There is a fairly good inverse correlation between permeability to water and threshold pressure. (3) All of the permeability and threshold pressure data was taken on these samples in a vertical direction. Because of the layered and laminated structure of these rocks, one would expect to find far different numbers in a horizontal direction. This has implications with regard to horizontal flow between adjacent sand lenses at the same depth, as opposed to vertical flow between underlying and overlying sand bodies. (4) Although a few of the barrier cores appeared marginally adequate to transmit gas between sand units based on the CTA data, whether they will do so in the ground is doubtful. (5) The material most likely to allow gas migration between sand lenses was that in the Coastal Zone. (6) It is far more likely that if gas migration occurs between sand lenses in the Multiwell, gas will move through the barrier material by way of natural or man-made fractures. Fracture permeability is probably many orders of magnitude higher than the ultra-tight shale and siltstone matrix, and threshold pressure in a fracture is quite low. The tests performed by IGT, however, were done on carefully chosen, unfractured core, and we have no information on the dynamics of gas movement through fractured stratigraphic barriers. The stratigraphic barrier material between sand lenses in the Mesa Verde at the Multiwell site would make an excellent caprock for an underground gas storage field. 11 figures, 2 tables. (ERA citation 09:030253)